Coal fly ash (CFA) is a solid particulate by-product of coal combustion that can be removed from the flue gas stream by cyclonic separation, electrostatic precipitation and bag house filtration. CFA may contain environmental toxins, such as arsenic, beryllium, boron, cadmium, chromium VI, cobalt, lead, manganese, mercury, molybdenum, selenium, strontium, thallium, vanadium, among other environmental toxins. In the past, coal fly ash was released into the atmosphere as a result of inadequate particulate removal from coal combustion flue gasses.
Coal-fired power plants now employ methods for capturing the CFA from the flue gas stream using various techniques, including cyclonic separation, flue gas desulferization units, electrostatic precipitation, and/or bag house filtration, among other techniques. The CFA is generally stored proximate to the coal power plants in wet or dry impoundments. Alternatively, the CFA is disposed in landfills. Under appropriate conditions, it is known to use the CFA as a supplemental cementitious material (SCM) in concrete mixes. The CFA includes pozzolanic materials, such as ceramic spheres (mainly silica and alumina). When used in concrete mixes, the pozzolanic materials can enhance the long term quality, durability, and compressive strength of the resulting concrete.
In addition to the toxic components of CFA described above, coal-fired power plants generate sulfur and nitrogen oxide (SOx and NOx) emissions. If released into the environment, these oxides form weak acids upon contact with surface waters or precipitation. Power plant operators often use activated carbon to absorb SOx and NOx, as well as other acid gasses and toxic pollutants such as mercury, thus reducing harmful emissions in the flue gas stream.
The activated carbon used to absorb these pollutants increases the overall carbon content of the solid particulate material, including CFA that is recovered from the flue gas. Federal regulations prohibit using the CFA in cement and/or concrete mixes if the carbon content exceeds approximately 6%, as determined by loss of weight upon ignition (>6% LOI).
One reason that the high carbon CFA cannot be used in concrete is that the carbon interferes with air entrainment (the intentional creation of tiny air bubbles in concrete), introduced to increase the durability of hardened concrete. Thus, the activated carbon used to clean the flue gas may render the recovered CFA unusable as supplemental cementitious material. This, in turn, can result in more CFA being stored at dry landfills or in wet slurry impoundments.
Accumulations of coal fly ash and associated bottom ash and boiler slags in landfills and wet impoundments constitute a major environmental hazard. These impoundments can fail, causing billions of dollars of damage in the process. In addition, toxic components of the CFA may leach into ground water when the CFA is stored in unlined impoundments. The ponds and impoundments where much of the CFA is stored by the operators of coal fed power plants are an increasing environmental concern. The Environmental Protection Agency has proposed rules to require that CFA not used in concrete be stored in lined landfills or other approved sites. Enforcement of the rules could greatly increase the cost of CFA disposal, thereby increasing the cost of energy generated from coal.
Gasification is a process wherein organic carbonaceous (mainly organic) materials are dissociated at high temperatures in an oxygen-starved environment to form a gas known as synthesis gas, or syngas, or producer gas. The syngas includes mainly carbon dioxide, carbon monoxide, hydrogen, methane and water vapor, as well as trace amounts of sulfur and other oxides.
If the thermal reactor is operated as a gasifier and is air fed (as opposed to oxygen fed only), the syngas stream also contains nitrogen gas. This latter form of syngas, which includes di-molecular nitrogen in relatively large quantities, is more specifically referred to as producer gas. However, according to common usage of terms, the gas phase product from the thermal reactor will be referred to as syngas throughout this application. Gasification is an efficient and relatively clean method of converting organic materials to energy, as compared to combustion or incineration.
The thermal reactor/gasifier is brought to operating conditions, including operating temperature, by combusting a suitable fuel source, such as natural gas or diesel fuel. The operating temperature is attained before the feed material is introduced into the gasifier. The air inflow rate, fuel moisture content, and fuel feed rates are tightly controlled to maintain the desired temperature and oxygen partial pressure for gasification to proceed efficiently.
In this regard, additional air may be provided to the thermal reactor, which operates as a gasifier, to increase the amount of oxidation that occurs. Additional air may be used when converting some feedstocks. In some circumstances, it may be preferable to use the injected air to combust most or all of the produced syngas before it leaves the thermal reactor. Alternatively, the syngas may be combusted in a separated oxidation chamber or steam boiler, or in a boiler to which a furnace has been affixed.
What is needed is an apparatus and method of reprocessing coal fly ash to recycle otherwise unusable high carbon CFA for use as an SCM.